The circle of Willis is a ring of arteries at the base of your brain that connects the brain’s two independent blood supplies into a single loop. It acts as a safety net: if one artery feeding the brain becomes narrowed or blocked, blood can reroute through the circle to reach areas that would otherwise lose their supply. Named after the English physician Thomas Willis, who published a detailed description of it in 1664, this small arterial ring plays an outsized role in protecting the brain from stroke.
How the Circle Is Built
Your brain receives blood from two systems. In the front, the two internal carotid arteries travel up through your neck and into the skull. In the back, the two vertebral arteries merge to form a single basilar artery. These front and back systems would normally operate independently, but the circle of Willis links them together through a series of connecting vessels.
Each internal carotid artery splits into an anterior cerebral artery (which supplies the front of the brain) and a middle cerebral artery (which supplies the sides). A short anterior communicating artery bridges the two anterior cerebral arteries, completing the front half of the ring. Meanwhile, the basilar artery splits into two posterior cerebral arteries, and a posterior communicating artery on each side links the internal carotid to the posterior cerebral artery. That completes the back half.
The result is a roughly hexagonal loop sitting in a small space at the base of the brain called the interpeduncular fossa. The optic chiasm, the point where the optic nerves cross, sits right in the front portion of the circle. The pituitary gland and several cranial nerves that control eye movement are also close neighbors.
Why It Matters: Collateral Blood Flow
The circle’s primary job is redundancy. If one of the major feeding arteries becomes severely narrowed or blocked, blood can flow through the ring in the opposite direction to compensate. For example, if the left internal carotid artery becomes obstructed, blood from the right carotid can cross through the anterior communicating artery to supply the left side of the brain. Similarly, blood from the basilar system can travel forward through the posterior communicating arteries to fill the gap.
This backup system has real clinical consequences. Patients who have a functioning anterior communicating artery and posterior communicating artery on the side of a severe carotid blockage have a lower risk of transient ischemic attacks (mini-strokes) and full strokes compared to patients whose connecting arteries are too small or absent. The anterior communicating artery is generally considered the single most important collateral route when a carotid artery fails, though the posterior communicating arteries also provide meaningful protection.
A well-functioning circle doesn’t just maintain blood pressure in the brain. It also appears to reduce harm from blood clots that break loose and travel into the brain’s vessels, because sufficient collateral flow can keep tissue alive even when a small vessel downstream gets plugged.
Most People Don’t Have a “Textbook” Circle
Here’s the surprising part: more than half of all people have some variation in their circle of Willis. A meta-analysis of anatomical studies found that roughly 68% of people show at least one departure from the classic textbook diagram. The most common variations involve the posterior communicating arteries, which may be abnormally small (hypoplastic) or missing entirely on one or both sides. About 19% of people have this on one side, and around 23% on both sides.
These variations are usually harmless and go unnoticed for a lifetime. But they become clinically relevant if you develop blockages elsewhere. A study of patients without prior stroke history found that people with an incomplete front portion of the circle had about 2.8 times the risk of a future stroke in the front of the brain. When both the front and back portions were incomplete, the risk climbed as high as 7 times normal. In other words, the circle’s protective value depends on whether your personal version of it is complete enough to reroute blood when needed.
Aneurysms and the Circle of Willis
The circle is also the most common site in the brain for berry aneurysms, small balloon-like bulges in artery walls. These tend to form at branch points where blood flow creates extra stress on the vessel wall. The three most frequent locations are near the anterior communicating artery, at the branching point of the middle cerebral artery, and where the internal carotid meets the posterior communicating artery. Aneurysms in the back of the circle, involving the vertebral or basilar arteries, account for fewer than 10% of cases.
Most berry aneurysms never rupture and are often discovered incidentally during brain imaging for other reasons. When one does rupture, it causes a subarachnoid hemorrhage, a type of bleeding around the brain that is a medical emergency. The circle’s location near the optic chiasm and eye-movement nerves means that both the aneurysm itself and any surgical repair carry a risk of affecting vision, depending on which part of the circle is involved.
Which Brain Regions Each Artery Feeds
The arteries branching off the circle each serve distinct territories. The anterior cerebral arteries supply the frontal lobes and the inner surfaces of the brain’s hemispheres, areas involved in decision-making, personality, and leg movement. The middle cerebral arteries, the largest branches, supply the outer surfaces of the brain including areas critical for language, sensation, and arm and face movement. The posterior cerebral arteries supply the occipital lobes at the back of the brain, where visual processing happens.
This territorial division explains why different strokes cause different symptoms. A blockage affecting the middle cerebral artery territory might cause weakness on one side of the face and arm along with difficulty speaking, while a posterior cerebral artery stroke is more likely to affect vision. The circle of Willis sits upstream of all these territories, which is exactly why its ability to redistribute blood flow is so valuable.
A Brief History of the Name
Thomas Willis, an English physician, published his landmark work “Cerebri Anatome” in 1664. The book described the brain and nervous system in unprecedented detail and remained influential for nearly two centuries. Willis wasn’t the first to notice arteries at the base of the brain; earlier, cruder descriptions already existed. But his account was thorough enough to become the definitive reference. The term “circle of Willis” didn’t appear in print until over a century later, in the 1774 edition of the Bibliotheca Anatomica, cementing his name in medical vocabulary permanently.